The present invention relates to a synchronization arrangement for a digital radio pager.
A digital radio pager is required to receive and process reliably broadcast digital signals which may be noisy and jittery at the point of reception. Moreover, it must be able to achieve reliable operation very quickly after initial start-up, or regain reliable operation after any interruption in the received digital data stream, e.g., when emerging from an area where radio reception is masked, such as in a tunnel or under a bridge. At the same time the receiver must utilize efficient battery saving techniques.
To appreciate the constraints placed upon a digital radio pager receiver consider the public radiopaging service currently under development by British Telecommunications. The full specification for the Standard Code Format is to be found in the Final Report of the British Post Office Code Standardisation Advisory Group (POCSAG). Suffice it to say that each address is transmitted only once and consists of a 32-bit digital code word transmitted in a batch of words, each batch comprising a synchronization code word followed by 8 frames each containing two code words. Each transmission starts with a preamble to permit the pagers to attain bit synchronization and to prepare them to acquire word synchronization. Since a pager may be switched on at any time, or emerge from a masked area at any time, it is desirable that it can attain bit and word synchronization, discriminating between true data and random noise, in a minimum time. The first complete transmission including associated preamble received after switch-on must be fully effective so far as the user is concerned.